The present invention relates generally to the electrolytic processing (anodizing, plating, etc.) of metals and specifically to the automated production of anodized coatings on aluminum, said coatings having superior hardness, thickness and a relative absence of surface and subsurface coating defects.
In the past it has been known that extensive process knowledge is necessary by an operator in order to rapidly generate thick, adherent and hard anodic coatings on aluminum and aluminum alloys. The basic process places the material to be coated in an acidic electrolyte and connects the material to the anode of a current source. An aluminum oxide coating will be formed on the aluminum material as current flows through the electrolyte and the material. The results of the anodizing process, such as the abrasion resistance, thickness, surface and subsurface coating defects, are the consequence of a number of different factors such as the chemistry of the electrolyte, electrolyte temperature, current density, the voltage excursion applied to the material and the aluminum alloy itself and temper thereof.
For example, using a sulfuric acid solution, low anodizing temperatures, ranging from 20.degree. to 40.degree. F., generally improve the abrasion resistance of the anodic coatings. However, low temperatures may reduce the anodizing rate and the maximum hard coating thickness obtainable with certain alloys such as 6061 depending upon the acid concentration and the current density utilized. In general, decreased temperatures increase the occurrence of local coating degradation known as "burning" which refers to a rapid and localized electrochemical milling of the part which renders the coating or part irreparable. Therefore, it can be seen that there is a trade off between coating abrasion resistance and the occurrence of burning when one chooses the electrolyte temperature. At temperatures above 50.degree. C., unacceptable, powdery anodic coatings are produced in many cases. However, with certain alloys (including 6061), hard anodic coatings are produced with an increase in anodizing rate depending upon the acid concentration and the current density utilized.
The processing current density also greatly influences the anodic coating abrasion resistance, anodizing rate and burning tendency. Constant current densities greater than 50 amperes per square foot produce powdery or burned coatings on most aluminum alloys, yet current density just below this limit produce the highest anodizing rate, depending again upon acid concentration and temperature. This general rule is complicated because heat treatments which increase the aluminum alloy hardness beyond the T6 condition, typically require even higher current densities for proper anodization, depending upon the alloy.
The acid concentration will always diminish during anodization. The operator must learn, from experience, how to compensate for the resultant effects. Additionally, as the coating thickness increases beyond about two mils, the anodizing rate generally becomes relatively slow and the coating abrasion resistance will decrease. Consequently, the operator of anodizing equipment needs much experience and extensive process knowledge in order to do an acceptable job. Table I is a summary of coating effects caused by variations in the anodization process parameters.
TABLE I ______________________________________ Process Response Trends Coating Maximum hard abrasion Anodizing coating Burning resistance rate thickness tendency ______________________________________ Decreased .uparw. .uparw. or .dwnarw. .uparw. or .dwnarw. .uparw. temperature Decreased .uparw. or .dwnarw. .dwnarw. .uparw. or .dwnarw. .dwnarw. current density Decreased .uparw. or .dwnarw. .dwnarw. .uparw. or .dwnarw. .uparw. acid con- centration ______________________________________
Where two opposite effects are given for a single parameter change, the trend direction is dependent upon the particular aluminum alloy and/or its heat treatment and/or the values of other anodizing parameters.
Other factors which are related to the above are the coating breakdown voltage (V.sub.b), the time dependence of the process voltage (V(t)) and the anodizing time (t). The coating breakdown voltage is defined as that alloy dependent voltage at which "burning" occurs. If the process voltage is maintained below the coating breakdown voltage, the coating thickness will be limited only by acid dissolution and degradation of the coating itself. These effects become more pronounced as the process voltage increases since heat is generated at the anodized surface in proportion to the power applied (the product of current and voltage). Therefore a low anodizing voltage is desired in order to maximize the coating abrasion resistance of any alloy.
Although V.sub.b, V(t) and t characterize the hard anodizing process, there is great difficulty in controlling V(t). The process voltage exponentially increases with respect to an increasing coating thickness (given a constant current). This increasing voltage in turn increases the power dissipated during the anodizing process which increases the localized heating, dissolution and degradation of the anodic coating. Therefore, it is desirable to control the hard anodizing voltage without damaging the coating abrasion resistance.
Alternating the anodizing current waveform during the anodizing process is well known and is disclosed in U.S. Pat. No. 3,983,014 to Newman, et al., entitled "Anodizing Means and Techniques". This variable polarity anodizing process solved a number of the conflicting problems and provided greater coating thickness at higher anodizing rates. Unfortunately, the above process has very short duration pulses and requires continuous operator attendance. The results were not readily reproducible because of the crude voltage control available. Also, no general technique was determined which would rapidly generate thick, adherent and hard anodic coating on all aluminum alloys.
Unless this variable polarity anodizing process is used, the anodizing voltage will, when uncontrolled, exceed the coating breakdown voltage after approximately forty minutes of processing. Consequently, the coating thickness is limited and unacceptable coatings or damaged parts are unavoidable unless the process is terminated after a relatively short anodizing time. As noted earlier, the coating properties are highly dependent upon the anodizing voltage itself. At first blush, it would appear that simply using a voltage controlled power supply would solve the above problems. However, as the dielectric coating thickness increases, there is an increasing voltage requirement to maintain the current flow in the process. If the current is allowed to diminish, the increased processing time in the acid electrolyte will degrade the coating abrasion resistance below acceptable standards (in particular, below MIL-A-8625 standards).
In attempting to optimize the hard anodization process for various alloys, it has been found that one pattern of waveform alterations (as in the variable-polarity anodizing process) produced favorable results with one aluminum alloy, but gave poor results with other alloys. Further, the coating breakdown voltage varies widely depending on the alloy being anodized. Consequently, the utilization of a set pattern of current waveform alterations gives uniformly poor results. Furthermore, it would be too costly to experimentally determine the proper individual waveform alteration pattern for each different alloy to be anodized.